1. Field of the Invention
The present invention relates generally to die contacts in integrated circuits, and more specifically to backside contact pads employed in lieu of conventional peripheral contact pads.
2. Description of the Prior Art:
Integrated circuits have conventionally employed contact pads on the "front" side of a wafer, or the side of a wafer on which devices are or were formed. The contacts pads, normally square-shaped and approximately 100 .mu.m on a side, are usually located around a peripheral portion of each die. Wires, typically gold, are then formed to connect each die contact pad to an appropriate pad in the packaging, which is generally electrically connected, in turn, to a pin.
Since die contact pads are generally formed using every metallization layer on the front side and are never formed over active devices, placement of the contact pads on the wafer's front side may increase the size requirement of the die. If a large number of contact pads are required and are placed only around the periphery of the die, the number, size and spacing requirements for the contact pads may dictate die size.
For some types of integrated circuits, placement of die contact pads on the front side of the integrated circuit also presents difficult processing challenges. An example is fingerprint acquisition circuits, which employ arrays of capacitive sensor electrodes coated with a dielectric onto which the finger is placed with the epidermis in contact with the dielectric. Since capacitance between two capacitive plates is inversely proportional to the distance between the plates, using the contacting dermal tissue itself as one capacitor plate and the sensor electrode as the other and then determining capacitance for each sensor electrode in the array, it is possible to locate the ridges and grooves of the fingerprint. Because physical contact with the finger is necessary, the hardest possible passivation dielectric coating over the capacitive sensor electrodes is required to prevent "scratch" damage to the sensor electrodes. The capacitive sensor electrodes must be formed in the last metallization layer, as would die contact pads if formed on the front side of the integrated circuit. Either selective formation of the passivation material over the sensor electrodes or selective removal of the passivation material over the contact pads would therefore be required, which may limit the materials which may be employed for the passivation or present other processing challenges. The hardest available passivation material may be effectively "non-etchable."
It would be desirable, therefore, to provide die contact pads in a location other than the front side of a wafer. It would further be advantageous if the die contact pads could be positioned to reduce die size requirements to the area required for active devices within the integrated circuit.